Internal combustion engines may be controlled in a variety of ways to provide acceptable driving comfort across a range of operating conditions while still meeting the performance demands of the driver. Some engines may utilize two or more modes of operation to achieve improved drivability and performance. As one example, one or more cylinders of an engine may transition between a spark ignition mode and a homogeneous charge compression ignition mode based, for example, on the amount of torque requested by the driver. As another example, engine output may be coordinated with the selective use of a secondary motor to achieve improved efficiency, drivability, and performance, such as is the case with a hybrid propulsion vehicle system.
However, during some conditions, the torque produced by the engine and/or motor may change rapidly due to a mode transition or a change in the vehicle performance requested by the driver. For example, if a rapid increase in engine torque occurs within a lash region of the transmission or other system of the vehicle driveline, noise and vibration harshness (NVH) or “clunk” may occur. In some cases, this clunk may be perceived by the driver where the transmission is transitioned too rapidly between a positive and negative torque transfer. As one example, a transition of one or more engine cylinders between combustion modes may cause a temporary torque transient that may increase the likelihood of clunk if the transition is performed within or near the lash region. Similarly, the addition and subtraction of torque from the driveline via a secondary motor may increase the likelihood of clunk where the transmission is operated near or within the lash region.
In one approach described herein, some of the above issues may be addressed by a vehicle control method for a vehicle including an internal combustion engine and an electric motor coupled to a torque converter, the torque converter having a speed ratio from torque converter output speed to torque converter input speed, the torque converter coupled to a drive wheel of the vehicle by a transmission, the method comprising selecting a rate of change limit based at least on a speed ratio across said torque converter input and output speeds; and adjusting an operating parameter of at least one of the engine and the electric motor to control a change in a combined output of the engine and electric motor to be less than said rate of change limit. In this way, by controlling the operation of the engine and/or motor, clunk can be reduced.
In another approach also described herein, some of the above issues may be addressed by a method for controlling a propulsion system for a vehicle including a transmission coupling an output shaft of the internal combustion engine to a drive wheel of the vehicle, wherein said transmission includes a lash region, the method comprising: adjusting an operating parameter of the engine so that at least one cylinder of the engine is transitioned between a first combustion mode and a second combustion mode; and varying a timing of said transition responsive to the lash region of the transmission. In this way, engine mode transitions may be scheduled in response to the operating state of the transmission, particularly the lash region of the transmission, so that clunk may be reduced.